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FreeBSD-SA-21:04.jail_remove                                Security Advisory
                                                          The FreeBSD Project

Topic:          jail_remove(2) fails to kill all jailed processes

Category:       core
Module:         jail
Announced:      2021-02-24
Credits:        Mateusz Guzik
Affects:        All supported versions of FreeBSD.
Corrected:      2021-02-19 01:22:08 UTC (stable/13, 13.0-STABLE)
                2021-02-19 21:53:07 UTC (releng/13.0, 13.0-BETA3-p1)
                2021-02-19 21:46:31 UTC (stable/12, 12.2-STABLE)
                2021-02-24 01:43:39 UTC (releng/12.2, 12.2-RELEASE-p4)
                2021-02-19 21:50:26 UTC (stable/11, 11.4-STABLE)
                2021-02-24 01:41:41 UTC (releng/11.4, 11.4-RELEASE-p8)
CVE Name:       CVE-2020-25581

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

The jail(2) system call allows a system administrator to lock a process
and all of its descendants inside an environment with a very limited
ability to affect the system outside that environment, even for
processes with superuser privileges.  It is an extension of, but
far more powerful than, the traditional UNIX chroot(2) system call.

The jail_remove(2) system call, which was introduced in FreeBSD 8.0,
allows a non-jailed process to remove a jail, which includes terminating
all the processes running in that jail.

II.  Problem Description

Due to a race condition in the jail_remove(2) implementation, it may fail
to kill some of the processes.

III. Impact

A process running inside a jail can avoid being killed during jail termination.
If a jail is subsequently started with the same root path, a lingering jailed
process may be able to exploit the window during which a devfs filesystem is
mounted but the jail's devfs ruleset has not been applied, to access device
nodes which are ordinarily inaccessible.  If the process is privileged, it may
be able to escape the jail and gain full access to the system.

IV.  Workaround

The problem is limited to scenarios where a jail containing an untrusted,
privileged process is stopped, and a jail is subsequently started with the same
root path.  Users not running jails are not affected, and the problem can be
avoided by not starting a jail with the same path as a previously stopped jail.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date and reboot.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

[FreeBSD 13.x]
# fetch https://security.FreeBSD.org/patches/SA-21:04/jail_remove.13.patch
# fetch https://security.FreeBSD.org/patches/SA-21:04/jail_remove.13.patch.asc
# gpg --verify jail_remove.13.patch.asc

[FreeBSD 11.x, FreeBSD 12.x]
# fetch https://security.FreeBSD.org/patches/SA-21:04/jail_remove.patch
# fetch https://security.FreeBSD.org/patches/SA-21:04/jail_remove.patch.asc
# gpg --verify jail_remove.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/13/                       894360bacd42f021551f76518edd445f6d299f2e
releng/13.0/                     9f00cb5fa8a438e7b9efb2158f2e2edc730badd1
stable/12/                                                        r369312
releng/12.2/                                                      r369353
stable/11/                                                        r369313
releng/11.4/                                                      r369347
- -------------------------------------------------------------------------

[FreeBSD 13.x]
To see which files were modified by a particular revision, run the following
command in a checked out git repository, replacing NNNNNN with the revision
hash:

# git show --stat NNNNNN

Or visit the following URL, replace NNNNNN with the revision hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

[FreeBSD 11.x, FreeBSD 12.x]
To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-25581>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-21:04.jail_remove.asc>
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